This invention relates to stable dispersions of polymers of conjugated dienes. More specifically, it relates to such dispersions which contain only low levels of surfactant.
It is known that polymers can be obtained by an anionic copolymerization of a conjugated diene compound and, optionally, an alkenyl arene compound by using an organic alkali metal initiator. Polymers have been produced which comprise these different monomers such as polystyrene and butadiene or isoprene or blocks thereof. These polymers may have configurations which are linear, radial or star, i.e. many arms radiating from a central core. In block copolymers, the proportion of thermoplastic blocks to elastomeric blocks and the relative molecular weights of each of these blocks is balanced to obtain a rubber having unique performance characteristics.
It has been found advantageous to prepare latexes of these polymers in order to obtain products with different properties. For instance, either protective coatings or adhesive films can be formed on surfaces by evaporating water from latexes containing high molecular weight polymers. Similar films of high molecular weight block copolymers are usually formed by hot melt or solvent evaporation. These latexes are comprised of the polymer, stabilizing surfactant(s), water and possibly coalescing solvents.
It has proved difficult to make such dispersions without using relatively high levels of surfactants, for instance 4 phr (parts per hundred rubber) sodium dodecyl sulfate (SDS), because styrenic block copolymers are incompatible with water and readily coagulate. In end uses such as adhesives and moisture resistant coatings, that much surfactant cannot be tolerated because adhesive tack is destroyed by the migration of surfactant to the surface and surfactant allows a pathway for moisture ingress in coatings. Adding functionality to the diene portion of the molecule has been tried as a way to reduce the amount of surfactant necessary to make dispersions of less than 1 micron particle size. For instance, polymers described in U.S. Pat. No. 4,578,429 (carboxylic acid or anhydride, such as maleic acid or anhydride, functionalized polymers) have been tried but these still require 2 phr of surfactant to achieve 1 micron particle size.
We have found that stable polymer dispersions with an average particle size of less than one micron can be made with the polymers of this invention. Such fine polymer dispersions are desirable because they do not coagulate, settle or cream quickly and fine particle sizes are known to be advantageous in the coalescence of homogeneous films.